Tech View: Cognitive Radio for Multimedia Home Networks
The Digital TV Transition, which marks the end of commercial analog TV broadcasts in the US, opens the door to a broad range of entirely new applications of wireless technology. By this June, or shortly thereafter, virtually all the TV channels now used for analog TV must cease transmission. The US will then be covered with a patchwork of white spaces, each one corresponding to the service area of an unused TV channel and interspersed among the service areas of digital TV channels that remain on the air. No license will be required to operate wireless devices in the white spaces. As long as users of white space frequencies follow some general rules designed to ensure they don’t interfere with existing licensed services (primarily digital TV), they are free to implement whatever activities and applications they wish. In striking contrast to previous practice, where lawmakers in effect decreed what services would be offered in the wireless marketplace, this new paradigm simply sets the rules for fair play, and then throws further development open to market forces.
Because of their excellent propagation characteristics, these newly available white space frequencies are considered ‘beachfront property’, and there is no lack of enthusiasm to use them to test new ideas. Applications ranging from large-scale Regional Area Networks (RANs) and mesh networks for Internet access to residential Local Area Networks for multimedia transport are all likely to be trialed in the near future. Given their potential to provide broadband access to underserved populations, especially in rural areas, white space RANs have received the lion’s share of public attention. Equally important, though apparently less newsworthy, is the possibility of using white space Local Area Networks (LANs) to enable whole-home coverage for broadband services such as AT&T U-verse. White space LAN technology will almost certainly be less expensive than the labor-intensive approach of physically rewiring a residence with broadband cable or fiber. Moreover, compared with the frequencies used by conventional wireless LANs (especially the 5-6 GHz band used by 802.11n), the white space frequencies offer superior transmission through interior walls and relative freedom from radio shadows caused by obstructions, which makes installation of a white space LAN a simple, straightforward do-it-yourself project.
A white space LAN supporting multiple streams of HDTV as well as conventional IP-based traffic such as data and VoIP is shown in Figure 1. The white space server, using one or more vacant TV channels, manages traffic flow within the residence, mediates broadband access, and houses a library of multimedia content available throughout the home.
The key technical challenge for white space technology is to devise techniques that will allow flexible operation in the white spaces without creating interference to licensed services elsewhere. The most promising approach is a new and still embryonic technology called cognitive radio; that is, a system concept in which a wireless device is aware of its radio environment and adjusts its behavior accordingly – in the present case to avoid interfering with broadcast TV. Initial application of cognitive radio to white space operation requires a geolocation database containing information about all licensed TV operations. Before transmitting, a white space device (or its controller in a master-slave configuration) must contact the database with information about its position, derived, for example, via GPS, and receive a list of available channels; that is, frequencies that may be used without causing TV interference. To provide added protection, a “belt and suspenders” approach is used, which requires that each white space device must scan its radio environment with a highly sensitive receiver and report to the geolocation database any detection of licensed operations.
As spectrum-scanning techniques gain in sophistication and reliability, white space networks may climb a step up the “cognitive ladder” and rely solely on spectrum sensing, with no control by a central database. In terms of system flexibility, the notion of autonomous rather than centralized control is extremely attractive. The downside of this approach, however, is that it is vulnerable to the hidden-node problem: a white space device, shielded from a licensed transmitter by a large obstacle such as a hill, might incorrectly conclude that a channel is available for its use, thus creating the potential for TV interference. Still to be determined, and the subject of much debate, is how sensitive the white space scanner needs to be, in order to ensure detection of TV signals, even when in a hidden-node situation. Cooperative sensing, which uses the spectrum sensors in all the devices in a network to improve aggregate sensitivity, may be required to achieve acceptable performance. Ongoing research in AT&T Laboratories is exploring new techniques for extracting the maximum benefit from this collective approach.
In addition to the legal requirement of non-interference with licensed services, white space networks must also meet the practical requirement self-coexistence; that is, non-interference (or at least tolerable interference) with other white space operations. In any locale there will be a limited number of vacant channels – perhaps upwards of twenty in rural areas, but no more than a half-dozen in large cities – so some form of channel sharing is needed. The most challenging situation arises in multiple dwelling units, where two independent white space LANs might be separated only by the wall between adjacent apartments. Frequency re-use, a concept borrowed from cellular mobile radio, is likely to be useful here. Adjacent white space LANs would operate on different TV channels, but a channel used by one LAN could be reused by another farther away. Thus an entire apartment building, for example, could be served by just a few channels re-used over and over again across the entire structure. Coordination of frequency re-use could be managed by means of a beacon-based protocol to advertise the presence of a white space device and provide a framework for negotiating system resources.
Residential white space networks offer the possibility of simple, inexpensive, whole-home multimedia coverage. Cognitive radio, though still in its infancy, is the key technology for addressing the basic challenges of interference control and self-coexistence. As white space networks evolve from laboratory prototypes to practical consumer products, additional technical problems will undoubtedly arise. We can be confident that cognitive radio, also evolving, will develop new capabilities to meet these new challenges.
Paul S. Henry is a Member of the Access Technology & Applications Research Division at AT&T Labs, where his interests focus on bringing high-speed Internet connectivity to homes and businesses. After receiving his Ph.D. in physics from Princeton University, Mr. Henry joined AT&T (Bell) Laboratories, where he has been engaged in research on communications circuits and systems as well as radio astronomy instrumentation. He has served on the editorial boards of IEEE publications and has published papers or patented inventions in several fields, including millimeter-wave radio techniques, cosmology, optical fiber and powerline communications, wireless systems and data security. He is a Fellow of AT&T and the IEEE and was the keynote speaker at Infocom 2002 (New York) and ICCCP 2005 (Muscat, Oman).
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